1. Field of the Invention
The present invention relates to an electronically controlled fuel injection and ignition system for an internal combustion engine, and, more particularly, to such a system providing fuel enriching priming pulses in difficult to start conditions.
2. Description of the Prior Art
Recent advances in microprocessor technology and fuel injection systems for internal combustion engines have enabled the utilization of microprocessor-based electronically controlled ignition timing and fuel injection systems for both two-stroke and four-stroke internal combustion engines. The electronic control unit (ECU) develops fuel injector control signals that control the amount of fuel injected during each revolution of the engine primarily as a function of throttle position and engine RPM and secondarily as a function of engine and ambient condition sensors including the engine temperature, ambient air temperature and barometric pressure. Depending on the variations in air temperature, throttle position, engine RPM, engine temperature, and barometric pressure, each sensor provides a factor which is selectively combined by the software in the electronic control unit to derive a fuel injection pulse width appropriate to the existing conditions. The engine temperature T.sub.E may be the crankcase temperature T.sub.C, particularly in two-cycle engines where the fuel-air mixture is scavenged through and warmed by the crankcase, and, in certain systems, engine coolant temperature T.sub.W.
The refinement of the algorithms used in such ECU based EFI systems has progressed considerably in the effort to improve starting ability and running at low and high speeds with cold and warm engines and under a wide range of ambient conditions. The operation of a two-cycle snowmobile engine equipped with such a system has been broken down into a number of phases including pre-starting, initial cranking, low speed running or idling after starting is achieved, cranking again if the engine is stopped or dies, acceleration after warm-up, and normal running after engagement of the drive clutch, and specific fuel injection pulse width algorithms have been developed to optimize performance in each phase and to inhibit abuse, e.g. acceleration of a cold engine.
In each of these phases starting with the cranking phase, typically a basic fuel injection pulse width is retrieved from one (or more) stored look-up table or map that provides basic pulse width values as a function of throttle position and engine RPM. The values of the fuel map are pre-programmed in memory and are selected by the ECU software each time the injection pulse width is to be calculated. Factors are derived from the other sensor signal values and are combined mathematically to either add or subtract to the basic pulse width to tailor the fuel/air ratio for the specific air pressure, air temperature and engine temperature to arrive at a corrected fuel injection pulse width. Generally speaking, the basic pulse width is widened for lower altitude, cold engine, cold inlet air, etc., and narrowed for a warm engine, high altitude, and high ambient temperature, etc., in order to maintain the correct fuel/air ratio despite ambient air density changes. Such an electronically controlled fuel injection system is disclosed in U.S. Pat. No. 5,074,271 as well as pending U.S. patent application Ser. No. 603,274, filed on Oct. 25, 1990, entitled FUEL INJECTION CONTROL SYSTEM FOR AN INTERNAL COMBUSTION ENGINE, all incorporated herein by reference.
The above-incorporated '274 application and '271 patent describe such fuel injection systems having parallel, alternate, low speed or "cold engine" fuel map from which a low speed injector pulse width that varies as a function of crankcase temperature is derived. Upon initial cranking to start the engine, the fuel pump will operate for 3-5 seconds to pressurize the injectors. Then a wide pulse width priming fuel quantity or pulse will be delivered by the injectors to aid starting. During the cranking phase and after engine starting, the normal and low speed pulse widths are calculated and injection takes place once during each revolution of the crankshaft or, in certain circumstances, during every other revolution of the crankshaft. The larger of the two pulse widths is used as the injection pulse width. As the engine warms up and time passes, the normal operation employing the normal pulse width takes over. In this fashion, the low temperature, cold engine starting and running fuel injection is enriched.
If, after having started, the engine stalls or is turned off, a timer is started that inhibits the 3-5 second pressurization of the injectors and the delivery of the prime pulse for a time period which is selected to prevent unnecessary priming of a warm engine. Moreover, as described in the '271 patent, a leaner enrichment injection pulse width is employed on restarting a stalled engine to lessen the possibility of flooding.
U.S. Pat. No. 5,038,740 describes priming algorithms that set a supplemental fuel pulse width in accordance with the formula Ti=Tpre.multidot.K.sub.TA. Kn, where Tpre is a basic preliminary injection quantity or pulse width that is related to crankcase temperature, K.sub.TA is an air temperature correction coefficient, and Kn is a correction coefficient, shown therein in FIG. 6C, that decreases from 1.0 in direct relation to the number of successive prime pulses delivered. In the '740 patent, the throttle opening is also monitored, and the supplemental enrichment pulses are delivered upon detecting a certain rate of change in the throttle valve occurring a minimum time after the injection of the preceding enrichment pulse, indicating that an attempt to start the engine is being made. As shown in FIG. 7, successive prime pulses are delivered each time the throttle opening rate of change criteria are met, that is, each time the driver tries to manually prime the engine, as if it were carbureted. While each successive enrichment pulse is decreased in width in accordance with Kn to diminish the possibility of flooding, the temperature related basic pulse width Tpre remains the same, and flooding may still occur. Moreover, the supplemental enrichment pulses are delivered in addition to the normal cranking phase fuel injection pulses.
Various other algorithms have been proposed to enhance the likelihood of successful engine starting while trying to avoid flooding. U.S. Pat. No. 5,009,211 provides a system responsive to each of the counted number of kick-start attempts to deliver successively smaller width fuel injection pulses to avoid flooding. These starting pulse widths are derived from a running or normal basic pulse width modified as a function of temperature and pressure factors similar to the algorithms described above but decreased in width as the successive number of kick start attempts increases.
All of these priming and fuel enrichment algorithms for ECU controlled EFI systems for snowmobiles are based on the assumption that the gasoline blend being used remains the same. The cold starting of electronic fuel injected snowmobiles, which may be exposed to extreme temperature ranges and may inadvertently be fueled with "summer blend" gasoline having a low Reed Vapor Pressure (RVP), remains difficult. If the engine fails to start, it has been the practice in some instances to turn the ignition key off or otherwise disconnect the power to the ECU to reset the priming function and to induce another prime pulse which has the same width as the preceding prime and to crank the engine again in the repeated attempt to start it. This attempt to fool the ECU system may cause the engine to flood rather than start, since the second (and subsequent) prime pulse is as wide as the initial prime pulse, and further inconvenience the driver.